1. Field of the Invention
This invention relates to a process for forming a silicon oxide layer on an integrated circuit structure. More particularly, this invention relates to a process for forming a silicon oxide layer on an integrated circuit structure using a hydridosilane-based coating material, such as hydrogen silsesquioxane, as a source of silicon oxide.
2. Description of the Related Art
In the formation of integrated circuit structures, various conducting, semiconducting, and insulating materials are utilized. It is common to utilize SiO.sub.2 as the insulation material, at least in part due to the ease of growing SiO.sub.2 over exposed portions of either the silicon substrate or polysilicon materials present in the integrated circuit structure. SiO.sub.2 is also formed on an integrated circuit structure by deposition techniques such as, for example, by chemical vapor deposition (CVD) processes.
However, there are certain applications such as, for example, planarization processes, where it is preferable to form a SiO.sub.2 insulation layer by first applying a flowable coating material over the integrated circuit structure comprising a material which is a SiO.sub.2 precursor. Very often such SiO.sub.2 precursor coating materials are organic based materials. Such materials, when applied as coatings and then decomposed to form the desired SiO.sub.2, sometimes also contain carbon residues. When a SiO.sub.2 layer formed by decomposition of an organic precursor also contains carbon, subsequent exposure to other processing materials, such as an oxygen cleaning gas, can result in a reaction between the carbon and the processing material. For example, oxygen may react with the carbon in the coating to form a gas, thereby leaving behind a roughened surface on the oxide layer.
Because of this problem with carbon remaining in some SiO.sub.2 coatings formed by decomposition of organic-based silica coating materials, the use of hydridosilane resins as coating material has been proposed. Hydridosilane resins, sometimes also referred to as hydrogen silsesquioxane or as "H-resin", have the formula (H/SiO.sub. 3/2).sub.n, when fully condensed and hydrolyzed, where n is generally about 10-1000. Exemplary H-resins, when not fully hydrolyzed or condensed, may have the formula HSi(OH).sub.x (OR).sub.y O.sub.z/2, in which x=0-2, y=0-2, z=1-3, x+y+z=3, and the average value of y over all of the units of the polymer is greater than 0. Each R is independently a 1-6 carbon organic group which, when bonded to silicon through the oxygen atom, forms a hydrolyzable substituent. Examples of R groups may include alkyls, aryls, and alkenyls. Such hydridosilanes or hydrogen silsesquioxanes (H-resins) silicon oxide precursors and their use as coating materials are more fully discussed and described in Ballance et at. U.S. Pat. No. 5,145,723, the disclosure of which is hereby incorporated by reference.
Hydridosilanes or hydrogen silsesquioxanes (H-resins) SiO.sub.2 precursors as coating materials on integrated circuit structures have been found to flow well over the surfaces to be coated, including stepped surfaces, and provide a high yield of SiO.sub.2, as well as providing an SiO.sub.2 layer with a low carbon content. The coating material, after initial drying to remove the solvent used in applying the coating to the integrated circuit structure, is then cured to form the desired SiO.sub.2 ceramic layer by heating it to a temperature of from about 200.degree. C. up to as high as about 1000.degree. C.
While the use of such hydridosilane or hydrogen silsesquioxane coating materials as precursors for the formation of SiO.sub.2 layers on integrated circuit structure has, therefore, met with some success, it has not been without its problems. In this regard, incomplete curing of the material, i.e., incomplete conversion of the dried coating material to SiO.sub.2, has been noted when the coating is applied over a stepped surface. In particular, when the coating is applied over closely spaced apart steps, or in a narrow trench, the coating material at the bottom of the trench does not completely cure. It has also been noted that even when the steps are spaced farther apart, the coating material in the bottom of the comer adjacent to the step does not completely cure.
These problems are illustrated in FIG. 1, wherein an integrated circuit structure is generally shown at 2 which includes closely spaced apart steps 10, 12, and 14 formed over a substrate 4, as well as steps 16 and 18 which are spaced farther apart. When an H-resin coating 20 is applied over integrated circuit structure 2 and dried, subsequent curing of the dried coating with heat alone results in the formation of SiO.sub.2 adjacent the surface, as denoted at 22 in FIG. 1. However, region 24 between closely spaced apart steps 10, 12, and 14 and the furthest from the surface, does not completely cure. It is also noted that even when the steps are spaced further apart, as shown with steps 16 and 18 in FIG. 1, the regions shown at 26 adjacent the corners of steps 16 and 18 (and also on the respective outside corners of steps 10 and 14), also do not completely cure.
The curing of H-resin such as hydrogen silsesquioxane is a reversible reaction which can be represented by the following equation: EQU Hydrogen silsesquioxane.revreaction.SiO.sub.2 +H.sub.2 .uparw.
Thus, the completeness of the reaction to form the desired SiO.sub.2 product depends upon the ability of the hydrogen gas to leave the coating material, thereby driving the above equation to the right, i.e., to complete formation of SiO.sub.2. While it is not the intention to be bound by any theories of why the coating material does not completely cure, during prior art curing processes, it is thought that the coating material in the deepest portion of the trenches formed in the substrate or between closely spaced apart steps, as well as in the comers adjacent steps, has a reduced diffusion angle or reduced volume above the trapped hydrogen gas through which the hydrogen gas may pass, thus resulting in an incomplete curing of such portions of the coating material.
In any event, this failure to completely convert all of the hydrogen silsesquioxane coating material to SiO.sub.2 can result in further release or emission of hydrogen gas from the coating during subsequent steps, including steps wherein the presence of a reducing gas such as hydrogen would not be desirable. Furthermore, such incomplete curing of the hydrogen silsesquioxane coating material results in the presence, on the integrated circuit structure, of material having a different coefficient of expansion, resulting in potential stresses; a different water content; and a material having different etch characteristics from the fully cured SiO.sub.2.
The aforementioned Ballance et at. U.S. Pat. No. 5,145,723 discusses the curing of the dried hydrogen silsesquioxane coating material to form SiO.sub.2 at a temperature ranging from about 20.degree. C. to about 1000.degree. C., preferably about 50.degree. C. to about 800.degree. C.; and suggests the use of reactive environments comprising air, O.sub.2, oxygen plasma, ammonia, and amines. According to Ballance et at., the curing is preferably carried out under a wet ammonia atmosphere to hydrolyze the Si--H bonds and then under a dry ammonia atmosphere to effect removal of any remaining Si--OH groups.
Hanneman et at. U.S. Pat. No. 5,118,530 also teaches the curing of hydrogen silsesquioxane at temperatures ranging from about 20.degree. C. to about 1000.degree. C. The patentees state that the heating can be conducted at any effective atmospheric pressure from vacuum to super atmospheric and under any effective gaseous environment such as those comprising air, O.sub.2, an inert gas such as nitrogen, ammonia, amines, etc., and further states that any method of heating such as the use of a convection oven, rapid thermal processing, or radiant or microwave energy is generally functional in the curing process. Hanneman et al. also prefer the use of a wet ammonia atmosphere to hydrolyze the Si--H bonds followed by a dry ammonia atmosphere to effect removal of any remaining Si--OH groups.
However, it has been found that even when using curing conditions, such as disclosed in the above-mentioned patents, complete curing of all of the coating material to form SiO.sub.2 is less than satisfactory. It would, therefore, be highly desirable to provide a process for curing a hydrogen silsesquioxane coating material, previously applied to an integrated circuit, structure which will result in the conversion of substantially all of the coating material to SiO.sub.2.